Effect of human neuronal tau on denaturation and reactivation of rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase

Human neuronal tau-40 (htau-40) has been used to study denaturation and ren aturation of rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase (GAPD H, EC 1.2.1.12). Inactivation of GAPDH incubated with tau was more distingu ishably detected than that of control GAPDH during thermal and guanidine hy drochloride (GdnHCl) denaturation. However, tau did not influence the activ ity of GAPDH at room temperature or in solution without GdnHCl. A marked ch ange in both the emission intensity and emission maximum of the intrinsic f luorescence at 335 nm of GAPDH with tau was observed when GdnHCl concentrat ion was 0.8 M, but that of the control without tau occurred in 1.2 M GdnHCl . The first-order rate of the decrease in the fluorescence intensity of the enzyme with tau was approximately twice as great as that of GAPDH without tau. Kinetics of inactivation of GAPDH with tau in 0.2 M GdnHCl was a monop hasic procedure, instead of the biphasic procedure followed by the control, as described before [He, Zhao, Yan and Li (1993) Biochim. Biophys. Acta 11 63, 315-320]. Similar results were obtained when the enzyme was thermally d enatured at 45 degrees C. It revealed that tau bound to the denatured GAPDH but not the native molecule. On the other hand, tau suppressed refolding a nd reactivation of GAPDH when this enzyme was reactivated by dilution of Gd nHCl solution. Furthermore, tau improved the aggregation of the nonnative G APDH in solutions. It suggested that tau acted in an anti-chaperone-like ma nner towards GAPDH in vitro. However, tau lost that function when it was ag gregated or phosphorylated by neuronal cdc2-like protein kinase. It showed that tan's anti-chaperone-like function depended on its native conformation .